Non-invasive imaging of cytosine deaminase (CD) gene expression in clinical cancer gene therapy trials with CD plus 5FC combination therapy would be of considerable value. It would allow us to monitor the location, magnitude, and persistence of CD gene expression over time. It would also allow us to assess whether the level of CD gene expression is adequate with respect to producing sufficient levels of 5FC metabolites, which are toxic to the tumor tissue and can produce a good therapeutic response. Previous attempts by other researchers to obtain images and measures of CD gene expression with positron emission tomography (PET) and radiolabeled [18F]5FC in tumors that express CD have failed. This is because [18F]5FU, which is produced by the CD enzyme from [18F]5FC, freely diffuses out of the CD-expressing tumor cells and does not concentrate within these cells. We propose to develop and validate a novel hybrid "reporter" gene that combines two pro-drug activating genes, cytosine deaminase (CD) and uracil phosphoribo-transferase (UPRT). The UPRT gene would act as both a reporter gene for CD and as a therapeutic gene by potential CD plus 5FC pro-drug activation therapy. The combined action of the CD and UPRT enzymes should allow PET imaging with activation therapy. The combined action of the CD and UPRT enzymes should allow PET imaging with [18F]5FC and magnetic resonance spectroscopy (MRS) imaging with non-radiolabeled 5FC. Namely, CD converts [18F]5FC into [18F]5FU, and UPRT converts it into [18F]5FU-monophospate ([18F]FUMP), which is "trapped", within the transduced cell and accumulates to a high levels. The high levels of [18F]5FUMP and cold 5FUMP can be imaged with PET and MRS, respectively. The rate of UPRT-catalyzed conversion of [18f]5fu INTO [18F]5FUMP is significantly faster than the rate of CD-catalyzed conversion of [18F]FC to [18F]5FC to [18F]5FU. CD-mediated catalysis is the rate-limiting step in accumulation of [18F]FUMP in transduced cells. Therefore, images of [18F]5FUMP accumulation would reflect CD expression in transduced tumors. This proposal represents a new approach that allows multi-modality imaging and monitoring of treatment efficacy in CD plus 5FC pro-drug activation gene therapy of cancer. The results of studies in tissue cultures and in experimental animal tumors will be immediately applicable to ongoing gene therapy trials involving the CD and/or UPRT genes. This approach will provide clinicians with a choice of two imaging modalities for detection of gene expression and for monitoring responses to gene therapy. Namely, PET and/or MRS imaging could be used separately or in combination.